docs(realtime): add realtime related documentation in docs.agentscope.io

**Background**

This issue tracks the documentation work for the realtime module in AgentScope 2.0 on [docs.agentscope.io](https://docs.agentscope.io/), covering:

- The overall design of `RealtimeModelBase` and its websocket lifecycle
- The internal event protocols: `ModelEvents` (model→agent, provider-normalized) and `ClientEvents` (client→agent, inbound)
- How realtime outputs are exposed via the unified `AgentEvent` (including the new realtime-specific event types `REALTIME_SESSION_*`, `USER_SPEECH_*`, `REPLY_INTERRUPT`, `INPUT_TRANSCRIPT_*`)
- The `RealtimeAgent` lifecycle (`start` / `stop`, the forward and model-response loops, tool-call round-trip)
- `DataBlock`-based audio payloads and `media_type` conventions
- API references and usage examples for each supported realtime model

This issue is blocked by #1689 and the realtime implementation issues.